The antenna is a centerpiece of any wireless system. With the proliferation of wireless systems, antennas become increasingly numerous and thus difficult to accommodate on any platform of limited surface. An obvious solution is to employ antennas that can handle multiple functions so that fewer antennas are employed on the platform. For example, a major automobile manufacturer has publicly announced its goal to reduce the two dozen antennas on some high-end passenger cars to a single multifunction antenna. For platforms from automobiles to cell phones, such a multifunction antenna must also have sufficiently small size and footprint, low production cost, ruggedness, and aesthetic appeal. For airborne platforms, a multifunction antenna must also have sufficiently small size and footprint and an aerodynamic shape with low profile.
FIG. 1 shows a table that summarizes common wireless systems available for implementation on automobiles, many of which are also available for mobile phones, personal computers, and other small or large platforms on the ground or in the air. This table is by no means complete, as more and more wireless systems are emerging, such as various mobile satellite communications systems, UWB (ultra-wideband) systems, etc. Nor is the table consistent with all the conventions, some of which change with time or vary with geographical locations. Additionally, wireless services are still expanding, so is the need for multifunction antennas.
Such multifunction antennas have been discussed in publications (J. J. H. Wang, V. K. Tripp, J. K. Tillery, and C. B. Chambers, “Conformal multifunction antenna for automobile application,” 1994 URSI Radio Science Meeting, Seattle, Wash., p. 224, Jun. 19-24, 1994; J. J. H. Wang, “Conformal Multifunction Antenna for Automobiles,” 2007 International Symposium on Antennas and Propagation (ISAP2007), Niigata, Japan, August 2007; J. J. H. Wang, “Multifunction Automobile Antennas—Conformal, Thin, with Diversity, and Smart,” 2010 International Symposium on Antennas and Propagation (ISAP2010), Macao, China, Nov. 23-26, 2010) and U.S. Pat. No. (5,508,710, issued in 1996; U.S. Pat. No. 5,621,422, issued in 1997; U.S. Pat. No. 6,348,897, issued in 2002; U.S. Pat. No. 6,664,932, issued in 2003; U.S. Pat. No. 6,906,669 B2, issued in 2005; U.S. Pat. No. 7,034,758 B2, issued 2006; U.S. Pat. No. 7,545,335 B1, issued 2009; U.S. Pat. No. 7,839,344 B2, issued 2010), which are incorporated herein by reference.
Since a multifunction antenna must cover two or more wireless systems, which generally operate at different frequencies, its advances have been marked by ever broader bandwidth coverage. Since the surface area on any platform, especially that ideal or suitable for antenna installation, is limited, a basic thrust for the configuration of multifunction antenna is for shared aperture, size miniaturization, and conformability with the platform on which it is mounted. The multifunction antenna has an inherent cost advantage, as it reduces the number of antennas employed; this advantage can be further enhanced if it is configured to be amenable to low-cost production techniques in industry. In this context two recent U.S. Patent Applications revealed techniques claimed to have these merits (Application No. 61/469,409, filed 30 Mar. 2011; application Ser. No. 13/082,744, filed 11 Apr. 2011), which are incorporated herein by reference. Both Applications are based on the deployment of ultra-wideband low-profile traveling-wave (TW) structures amenable to planar production techniques.
It is noted that the two types of multifunction antennas addressed in these two Patent Applications have different spatial radiation patterns. Antennas in Application No. 61/469,409 radiate a unidirectional hemispherical pattern, while antennas in application Ser. No. 13/082,744 radiate an omnidirectional pattern. This Application discloses a class of multifunction antennas that radiate both unidirectional and omnidirectional patterns needed by some or all satellite and terrestrial services, respectively, as summarized in FIG. 1, by employing a plurality of different TW structures.
In prior art, a technique to reduce the size of a 2-D surface TW antenna is to reduce the phase velocity, thereby reducing the wavelength, of the propagating TW. This leads to a miniaturized slow-wave (SW) antenna (Wang and Tillery, U.S. Pat. No. 6,137,453 issued in 2000, which is incorporated herein by reference), which allows for a reduction in the antenna's diameter and height, with some sacrifice in performance. The SW technique is generally applicable to all TW antennas, those with omnidirectional and unidirectional radiation patterns.
The SW antenna is a sub-class of the TW antenna, in which the TW is a slow-wave with the resulting reduction of phase velocity characterized by a slow-wave factor (SWF). The SWF is defined as the ratio of the phase velocity Vs of the TW to the speed of light c, given by the relationshipSWF=c/Vs=λo/λs  (1)where c is the speed of light, λo is the wavelength in free space, and λs is the wavelength of the slow-wave at the operating frequency fo. Note that the operating frequency fo remains the same both in free space and in the slow-wave antenna. The SWF indicates how much the TW antenna is reduced in a relevant linear dimension. For example, an SW antenna with an SWF of 2 means its linear dimension in the plane of SW propagation is reduced to ½ of that of a conventional TW antenna. Note that, for size reduction, it is much more effective to reduce the diameter, rather than the height, since the antenna size is proportional to the square of antenna diameter, but only linearly to the antenna height. Note also that in this disclosure, whenever TW is mentioned, the case of SW is generally included.
With the proliferation of wireless systems, antennas are required to have increasingly broader bandwidth, smaller size/weight/footprint, and platform-conformability, which is difficult to design especially for frequencies UHF and below (i.e., lower than 1 GHz). Additionally, for applications on platforms with limited space and carrying capacity, reductions in volume, weight, and the generally consequential fabrication cost considerably beyond the state of the art are highly desirable and even mandated in some applications. The present class of multifunction antennas discloses techniques to address all these problems.